1. Field of the Invention
This invention relates to a toner for developing an electrostatic latent image formed by electrophotography, electrostatic recording or electrostatic printing and to a toner cartridge containing the toner. The present invention is also directed to an image forming method, a process cartridge and image forming apparatus using the toner.
2. Discussion of the Background
Various electrophotographic image forming methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 49-23910 and 43-24748. The methods typically include the following steps:
(1) the surface of an image bearable member such as a photoconductor is charged (charging process);
(2) the image bearable member is exposed to light to form an electrostatic latent image thereon (latent image forming process);
(3) the latent image is developed with an electrophotographic toner to form a toner image on the image bearable member (developing process);
(4) the toner image is transferred onto a receiving material (transferring process); and
(5) the toner image on the receiving material is fixed by application of heat, pressure, solvent vapor, or combination thereof to prepare a copy image (fixing process).
The method for developing electrostatic latent images is broadly classified into the following methods:
(1) a wet developing method using a liquid developer including a fine color pigment or dye dispersed in a liquid; and
(2) a dry developing method such as a cascade method, a magnetic brush method and a powder cloud method, which uses a developer (toner) including a colorant such as carbon black dispersed in a binder resin. Recently the dry developing methods are widely used.
As the method for fixing toner images, methods using a heat roller are widely used because of good energy efficiency thereof. Recently, there is an increasing demand for an image fixation method which requires as small a heat energy as possible to meet with high speed reproduction and saving of energy. In the DSM (Demand-side Management) program in 1999 of IEA (International Energy Agency), requirements for next generation image forming apparatuses are described. It is described in the requirements that the warming-up time should not be greater than 10 seconds and the power consumption in a waiting state, which changes depending on the copying speed, should not be greater than 10-30 watts in a copying machine having a copy speed not less than 30 cpm (copies per minutes). In order to meet these requirements, the power consumption of copying machines must be dramatically reduced.
In attempting to meet the above requirements, fixing devices using a fixing element such as a heat roller, which has a low thermal capacity and which has a quick temperature rising time, have been researched. However, the devices do not bring about the desirable effects.
In order to satisfy the requirements for minimizing the energy consumption of image forming apparatus while minimizing the warming-up time, it is essential to develop a toner which can be fixed at a low temperature.
The use of a polyester resin, which has better low temperature fixation property and better heat resistant preservability as compared with the conventional styrene-acrylate copolymer, as a binder resin of a toner has been proposed (Japanese Laid Open Patent Publications Nos. 60-90344, 64-15755, 2-82267, 3-229264, 3-41470 and 11-305486). Further, in an attempt to improve low temperature fixation properties, Japanese Laid Open Patent Publication No. 62-63940 proposes the use of a non-olefinic crystalline polymer and Japanese Patent No. 2931899 and Japanese Laid Open Patent Publication No. 2001-222138 propose the use of a crystalline polyester.
The above toners, however, are not fully satisfactory to meet the requirements of the DSM program.
There is provided in accordance with the present invention a toner for developing electrostatic latent images, comprising a colorant, a releasing agent and a binder resin, wherein said binder resin comprises a first, crystalline polyester resin, and a second, amorphous polyester resin having a softening point higher than that of said first polyester resin and being incompatible with said first polyester resin.
In another aspect, the present invention provides a toner cartridge containing the above toner.
The present invention also provides an image forming method, comprising developing an electrostatic latent image on an latent image-bearing member with the above toner.
The present invention further provides a process cartridge freely detachable from an image forming apparatus, comprising a photoconductor, and at least one means selected from the group consisting of a charger, image exposure means, developing means including the above toner, image transfer means, and cleaning means.
The present invention further provides an image forming apparatus comprising a developing unit for developing an electrostatic latent image on an latent image-bearing member with the above-described toner.
In order to improve low temperature fixation properties of a toner, it is necessary to control the thermal characteristics of the binder resin thereof. However, when the glass transition point (Tg) of the binder resin is excessively low, the heat resistant preservability of the toner is adversely affected. When the softening point of the binder resin is lowered by reducing the molecular weight thereof, a problem of hot offset (adhesion of the toner to the heat roll) is caused. Thus, it is difficult to obtain a toner having both excellent low temperature fixation efficiency and excellent offset resistance by controlling the thermal characteristics of the binder resin.
It has been found that the low temperature fixation properties can be improved by using a crystalline polyester resin. When heated above its grass transition point, such a polyester resin can cause a phase change from solid to melt at a relatively low temperature and the melt viscosity thereof quickly decreases. As a consequence, fixation to a transfer sheet such as paper proceeds efficiently. Since the grass transition point and the softening point of the crystalline polyester resin are low, the toner containing same has good low temperature efficiency. It has been also found that when such a crystalline polyester is used in conjunction with another, non-crystalline polyester resin having a higher softening point than the crystalline polyester resin, the two polyester resins form discrete domains such as sea-islands structure, so that the properties inherent to the two types of the polyesters can be suitably exhibited. Thus, the hot offset resistance of the resulting toner is improved because of the elasticity attributed to the non-crystalline polyester resin (second polyester resin) while retaining the desired low temperature fixation efficiency attributed to the crystalline polyester resin (first polyester resin).
It is an object of the present invention to provide a toner for developing an electrostatic latent image, which has excellent low temperature fixation efficiency and good offset preventing properties.
Another object of the present invention is to provide a toner of the above-mentioned type which has good preservability and which gives high grade toner images with excellent reproducibility.
It is a further object of the present invention to provide a method which permits the formation of a toner image on an image receiving sheet at a low fixing temperature and in a wide fixing temperature range without causing a hot offset problem.